As to polymer electrolyte fuel cells (PEFC), catalyst layers and gas diffusion layers are joined in that order to both external surfaces of an electrolyte membrane, where an electric power generation occurs, to thereby form a membrane electrode assembly, the membrane electrode assembly is held between a pair of separators to form a single cell, and a plurality of single cells are stacked to thus form a polymer electrolyte fuel cell.
In recent years, proton-conductive ion exchange membranes are used for electrolyte membranes. In particular, since cation-exchange membranes which include a perfluorocarbon polymer having a sulfonic group are superior in basic properties, such cation-exchange membranes have widely been studied. In addition, outer peripheries of electrolyte membranes are often supported by resin frames.
Moreover, a recessed groove is disposed in an inner portion of a separator. The groove serves as a gas flow channel when a membrane electrode assembly is disposed against the separator. Furthermore, a gasket may be disposed between the end of the separator and the electrolyte membrane in order to secure a gas sealing capability, i.e. in order to prevent outside leakage of a fuel or oxidant gas. In this way, the gasket exists between the separator and the electrolyte membrane, and thus, plays a role in sealing the gas flow channel from the outside.
When a fuel gas containing hydrogen, and an oxidant gas containing oxygen, such as the air, are supplied to a polymer electrolyte fuel cell in the above-described structure, the fuel gas is electrochemically reacted with the oxidant gas through the electrolyte membrane. By use of this principle, the polymer electrolyte fuel cell can simultaneously generate electric power, heat and water.
In a polymer electrolyte fuel cell, the following reactions occur, thereby generating electrical energy.At an anode: H2→2H++2e−  (1)At a cathode: ½O2+2H++2e−→H2O  (2)
However, in a conventional polymer electrolyte fuel cell, cross leakage of the gas may occur from a minute gap between the electrolyte membrane and the frame. The term “cross leakage” refers to a phenomenon in which a portion of the gas, which has been supplied to the inside of the cell, passes through the narrowest gap caused between the inner periphery of the frame and the electrode, and thus, the gas leaks from either of the anode side or the cathode side to the other side.
In order to improve a power generation efficiency in fuel cells, it is required to reduce such cross leakage of the gas.
As methods for solving the above-described problem, a technique in which an imperforate sheet is disposed inside the electrolyte membrane, and a technique in which the frame is formed by injection molding have been proposed (for example, see Patent Literatures 1 and 2). In addition, techniques described in Patent Literatures 3 to 7 have also been known.
FIG. 8 is a schematic diagram of a single cell in a conventional fuel cell.
In Patent Literature 1, a domain 1 (105) which has proton conductivity is present throughout an electrolyte membrane 100 in the thickness direction, while a domain 2 (106) which does not have proton conductivity and which is located around the outer peripheral portion of the domain 1 (105) is present throughout the membrane in the thickness direction by disposing an imperforate sheet therein. Further, portions from the outer peripheries of catalyst layers 101 to the outer peripheries of gas diffusion layers 102, where both the layers are disposed at both surfaces of the electrolyte membrane, are located in the domain 2 (106). In addition, a technology in which gaskets 104 are disposed therein to reduce cross leakage of the gas is disclosed in Patent Literature 1. A pair of separators 103 is also disposed at both sides of the gas diffusion layers 102.
FIGS. 9A, 9B and 9C are schematic illustrations of a method for producing a membrane electrode assembly for a conventional fuel cell.
Patent Literature 2 discloses a technology in which a frame 111b is formed by injection molding such that the inner peripheral portion of a membrane electrode assembly 110 is disposed inside a frame 111a, and thus, adhesiveness between peripheral areas of the membrane electrode assembly 110 and frames 111a and 111b is improved, thereby reducing cross leakage of the gas (FIGS. 9A to 9C).